Prosecutors in George Zimmerman's murder trial assailed his credibility today as they prepared to wind down their case in the shooting death of teenager Trayvon Martin.

Zimmerman, who is charged with second degree murder in the Feb. 26, 2012 shooting, maintains he shot Martin in self-defense as they struggled over Zimmerman's pistol.

Anthony Gorgone, a Florida Department of Law Enforcement crime lab analyst, testified that Martin's DNA was not found on the gun grip.

"You were able to exclude Trayvon Martin having DNA on the pistol grip, is that correct?" asked prosecutor Bernie de la Rionda.

"Yes," said Gorgone.

This appeared to contradict testimony Tuesday by a witness who said Zimmerman told him that Martin had grabbed the gun. However, in cross examination Gorgone admitted that it's possible Martin did touch the gun.

Gorgone's testimony followed that of a professor who said he taught Zimmerman criminal litigation in 2010 at Seminole State College. Capt. Alexis Francisco Carter told the court that the course included Florida's controversial "Stand Your Ground law and he gave Zimmerman an A.

The prosecutor's use of Carter's testimony was also aimed at undermining Zimmerman's credibility because he has denied prior knowledge of self defense laws.

When asked by Fox News' Sean Hannity last year "prior to this night, this incident, had you even heard stand your ground?"

Zimmerman responded, "No Sir."

Again Hannity asked, " You have never heard about it before?"

Again, "No." This interview was played in front of the jury earlier in the trial.

The prosecution is expected to rest its case soon, possibly Friday when the trial resumes. The prosecutors' final witness is expected to be Sybrina Fulton, Martin's mother.

Carter's testimony was made possible after Judge Debra Nelson handed the prosecution a legal victory before court resumed today by allowing Zimmerman's criminal justice course load, and his failed application for a Virginia Law enforcement job, to be entered into evidence. The ruling opened up Zimmerman's past to scrutiny.

It was the first time in eight days of testimony that jurors heard about Florida's "stand your ground" law.

Zimmerman had waived a pre-trial immunity "stand Your ground" hearing before a judge, which would have exonerated him from all future criminal and civil proceedings if a judge ruled in his favor. Zimmerman's attorney had argued that the former neighborhood watch captain wanted this case decided by a jury.

During cross-examination, defense attorney Don West asked Carter to give the jury an impromptu lecture on self defense. Carter noted that in a fight an initial aggressor can become the victim.

"It's imminent fear, so the fact alone that there isn't an injury does not necessarily mean that the person doesn't have a reasonable apprehension of fear," Carter told the court. "The fact that there were injuries has a tendency to show or support that that was an apprehension of fear."

West hammered at the point.

"You don't have to wait until you are almost dead before you can defend yourself?" asked West.

"No, I would not advise to do that," responded Carter, causing Zimmerman to laugh.

Carter's testimony during the cross helped blunt the prosecution's attempt for the second straight day to raise questions about Zimmerman's credibility.

On Tuesday, a medical examiner who reviewed video and photographs of Zimmerman's injuries suffered during his fatal confrontation with Martin called Zimmerman's wounds "insignificant" and "non-life threatening."

Dr. Valerie Rao testified that Zimmerman was struck as few as three times by Martin during the fight. She also asserted his head may have only been slammed on the concrete a single time. Zimmerman said Martin repeatedly slammed his head on the concrete.

"Are the injuries on the back of the defendant's head consistent with one strike against a concrete surface?" asked prosecutor John Guy yesterday.

"Yes," Rao said.

Rao's testimony could contradict Zimmerman's assertion that he was involved in a potentially life-threatening struggle with the Florida teenager.

Zimmerman claims he shot Martin in self defense as Martin repeatedly banged his head against the pavement and reached for Zimmerman's gun.